1. Field of the Invention
The present invention relates to a method and apparatus for detecting waveform degradation due to chromatic dispersion of an optical fiber in an optical transmission system, and also relates to a method and apparatus for compensating for such waveform degradation.
2. Description of the Related Art
In optical transmission systems, as transmission speeds increase, waveform degradation due to chromatic dispersion of optical fibers becomes a practical concern, and techniques for compensating for such waveform degradation become necessary. In the prior art, a compensating technique has been known that involves inserting a dispersion-compensating fiber having a dispersion value opposite in sign to the dispersion value of a transmission line.
This prior art technique has had the following problems.
The effect of optical fiber dispersion changes depending on temperature, transmission distance, characteristics of the fiber installed, etc., but, since practical systems do not have means for detecting the degree of dispersion-induced waveform degradation during system operation, it is difficult with the prior art to make optimum settings individually according to the system characteristics while the system is in service. PA1 Dispersion-compensating fiber has such shortcomings as high cost, large size, and high insertion loss. PA1 In ultra-high speed regions of about 10 Gb/s, electronic circuits must be implemented as integrated circuits, but resistors and capacitors capable of being varied over a wide range are difficult to implement on an IC. PA1 Increasing the operating bandwidth is difficult because of the bandwidth limiting by the bandwidth control circuit itself.
Further, controlling the frequency bandwidth of an equalizing amplifier circuit in an optical receiver in order to compensate for waveform degradation creates the following problems.